Update COPYING with 3-clause BSD.
Update COPYING with 3-clause BSD.

Closes #13.
Forty-Bot/libfxcg,Forty-Bot/libfxcg,Forty-Bot/libfxcg,Jonimoose/libfxcg,Jonimoose/libfxcg,Forty-Bot/libfxcg,Jonimoose/libfxcg